1. Field of the Invention
The present invention relates to a valve seat insert to be used in cylinder heads for piston engines and a cylinder head with the valve seat insert, in particular, to a combined valve seat insert consisting of a ceramic member and a metallic member and a cylinder head for a piston engine with the combined valve seat insert.
The valve seat insert is a cylindrical or ring shaped member and is used by securely fitting to the valve seating portion of the cylinder head of a piston engine in order to keep a pressure in the cylinder when a valve is seated on the seat insert.
2. Related Art Statement
Heretofore, valve seat inserts entirely made of ceramic material have been proposed in view of high wear resistivity and heat resistivity, as shown in Japanese Patent Application Laid-open Publication No. 59-162345 and Japanese Utility Model Application Laid-open Publication No. 60-12603.
The Japanese Patent Application Laid-open Publication No. 59-162345 discloses a valve seat made of partial stabilized zirconia (PSZ), while the Japanese Utility Jodel Application Laid-open Publication No. 60-12603 discloses a valve seat made of ceramic material for performing a part of a combustion chamber and a shock absorbing mechanism for releasing an impact load caused by striking a valve made of ceramic material against the valve seat.
The partial stabilized zirconia disclosed in the Japanese Patent Application Laid-open Publication No. 59-162345 has a low heat conductivity and a high heat insulating characteristic, but has a high heat expansion coefficient so that when the valve seat entirely made of partial stabilized zirconia is used under a condition wherein a temperature gradient exists, the valve seat will be destroyed by heat stress. Furthermore, the valve seat entirely made of ceramic material must be carefully handled when fitting into the cylinder head since the ceramic material lacks toughness. While, the valve seat disclosed in the Japanese Utility Model Laid-open Publication No. 60-12603 is entirely made of ceramic material and has a complicated shape in order to accommodate the shock absorbing mechanism. Such a complicated valve seat is difficult to product and has a problem in reliance for use, since ceramic material is difficult to machine and lacks toughness. Furthermore, such a valve seat entirely made of ceramic material is difficult to assemble automatically since ceramic material lacks toughness and requires careful handling when fitting into the cylinder head.
In general, the cylinder head is made of metallic material having a coefficient of thermal expansion higher than that of ceramic. Therefore, when the cylinder head provided with the valve seat entirely made of ceramic is heated, the combined strength between the cylinder head and the valve seat is decreased by the difference of the thermal expansion thus resulting in the valve seat comes loosening and falling out of position.
It can be understood from the above that the valve seat whole of which is made of ceramic is not practicable.